1. Field of the Invention
This invention relates to a trimming resistor element for use in a microelectronic circuit such as a thin film or a thick film integrated circuit, and more particularly to a trimming resistor element used for effecting functional trimming by use of a laser beam.
2. Description of the Related Art
Recently, in semiconductor integrated circuits and hybrid integrated circuits, functional trimming has received attention as a means of achieving highly precise output characteristics.
Since light is used for trimming by the laser beam, the trimming may be effected on an object to be trimmed which is electrically separated from the trimming device. For example, when a resistor is used as an element for determining the output characteristic of a circuit, the resistance thereof is previously set to a roughly determined initial value. First, the circuit is set into an operative condition before starting the adjustment of the resistance. Then, the resistance is adjusted by partially cutting off or processing the resistor element by application of a laser beam until a desired output characteristic is obtained, while the output characteristic of the circuit is observed. In this way, a highly precise output characteristic can be attained. This method is called functional trimming.
In general, the method of adjusting the resistance of a resistor element by use of functional trimming is divided into two methods as shown in FIGS. 5 and 6.
The method shown in FIG. 5 is effected by forming a groove or grooves in the trimming resistor element in order to adjust its resistance. In FIG. 5, reference numerals 1 and 1* denote electrodes and reference numeral 2 denotes a resistor film. When groove 3 is formed in the direction indicated by the arrow by a laser beam, the density and direction of electric forces between electrodes 1 and 1* change and as a result change the resistance between electrodes 1 and 1* (Refer to Wes Mickanin et al., "LWT RESISTOR DESIGN AND CIRCUIT CONSTRUCTIONS FOR PRECISION COMPONENTS" pp. 213-217, Tektronix Inc.).
The other method, shown in FIG. 6, is effected by cutting off at least part of a resistor network. In FIG. 6, an example of a trimming resistor element in which the resistor network is formed in a lattice configuration is shown. As resistor films 4a, connected in parallel with resistor film 4, are sequentially cut off by a laser beam, the resultant resistance, as viewed from electrodes 3 and 3*, changes. Like the case shown in FIG. 5, the density and direction of electric forces between electrodes 3 and 3* change. In this case, the resistance is adjusted by actually and selectively removing parallel-connected resistor films 4a from the resistor network.
The method of FIG. 5 in which the resistance is adjusted by forming a groove in the resistor film is a widely used method. However, the method has a serious problem in that the resistor is subject to aging, that is, the resistance thereof varies with time. It is well known in the art that a minute crack (which is hereinafter referred to as a microcrack) 3a is formed in the cut-away portion when a groove is formed by the laser beam. The microcrack may become greater due to application of a thermal or mechanical stress which affects the resistance of the resistor after the resistor is completely manufactured. Further, if the microcrack absorbs moisture and the distribution of electric forces in the microcrack portion is changed by the moisture absorption, the resistance of the resistor is changed with time. In most circuits in which the resistance adjustment is effected by functional trimming to achieve highly precise output characteristics, aging of the resistance may become a fatal problem.
On the other hand, in the method of FIG. 6, in which the circuit network is partially cut off, the cut-off resistor is substantially removed from the circuit network so that no current will flow through the parallel cut-off resistor film. Therefore, even if a microcrack occurs in the cut-off portion, the resistance of the trimming resistor will not subject to aging. However, the method has a different problem. The problem is explained with reference to FIG. 7.
FIG. 7A is a view of a trimming resistor network (trimming resistor element) as viewed from above, in the same manner shown in FIG. 6. Protection film 5 is generally formed on the trimming resistor element. When a laser beam is applied to the trimming resistor element through the protection film in order to selectively cut off parallel resistor films 4a, part of the laser beam is absorbed into the protection film. When this occurs, power necessary to achieve the cut-off operation is less-likely to be transmitted to the resistor element. Therefore, it is necessary to remove part of the protection film lying on the region to which the laser beam is applied. Thus opening 51 is made in the region.
FIG. 7B is a cross sectional view of part of opening 51 taken along line X--X* of FIG. 7A. Parallel resistor films 4a are formed on oxide film 8 of semiconductor substrate 7. When the protection film is partially removed, a corresponding portion of oxide film 8 lying under resistor film 4a is etched out and depressions are formed directly under resistor films 4a as shown by reference numeral 52.
If laser trimming is effected under this condition, the laser beam will be applied to the stepped portions of the oxide film immediately after the resistor films are melted away by application of the laser beam. As a result, the laser beam is refracted or diffracted by the stepped portions of the oxide film so that the laser beam may be locally focused on some regions, thereby causing silicon substrate 7 to be damaged.
Further, another problem is that an etchant, for etching the protection film or patterning electrodes, remains on the depressions 52 and makes it difficult to provide highly reliable products.
That prior art method of, shown in FIG. 5, forming a groove which is included in the conventional methods of adjusting the resistance of the resistor has a problem in that a microcrack occurs in the current path of the resistor film, causing the resistance thereof to vary with time. Further the prior art method, shown in FIG. 6 of selectively cutting off the resistor network has problems in that the reliability may be degraded by the damage caused in the substrate in the cut-off operation, and a residue of etchant used for removing the protection film may remain on the oxide film.